If you want to go into details, the full reliable polyfill is hard to write down due all inconsistencies across JS engines out there where the __proto__ setter cannot be reused which means it's not possible to trust this magic over objects created from null.
All you need to do is forget __proto__ and use the suggested polyfill until the day __proto__ can simply disappear from those specs pages.
Enjoy!
This is a full polyfill with some extra info you might want to analyze, whenever ployfill is strictly true or false.